Synopsis: Our long-term objective is to determine the roles of ER stress in the heart. Using a genomics approach we identified a group of genes induced by the ATF6 branch of the ER stress response (ERSR) that encode proteins predicted to have para/autocrine effects on the heart. These proteins, which we call ER stress-inducible cardiomyokines (ERS-CMKs), are unique, since they are synthesized in, and secreted from the heart during stresses, e.g. ischemia, that impair synthesis and release of most other proteins. The focus of this proposal is the ERS-CMK, mesencephalic astrocyte-derived neurotrophic factor (MANF), which is novel since it functions intra- and extracellularly to affect cardioprotection. In this proposal cardiomyokines (CMKs) are defined as proteins secreted by the heart that may exert para/autocrine effects, in part, by direct binding to heart cells, as well as binding to resident and, perhaps non-cardiac-derived stem cells to affect their function. Most CMKs are synthesized and folded in the rough ER, routed to the Golgi, then to secretory vesicles before secretion. In the absence of ER stress, conditions are optimal for expression, folding and, thus, secretion of functional CMKs. However, some stresses impair protein folding in the ER, activating ER stress, which decreases CMK transcription, translation, folding and secretion, thus leading to a loss of CMK function. ER stress leads to activation of ATF6, a nodal sensor of ER stress that increases transcription of many known ERSR genes that encode ER-targeted proteins that directly augment ER-protein folding. We showed that the ATF6 branch of the ERSR, which is activated during ischemia, protects the heart from damage during ischemia and reperfusion, ex vivo and in vivo. Microarray analyses of mouse hearts revealed numerous ATF6-inducible genes that encode proteins predicted to be ER-targeted and secreted (ERS-CMKs). One ERS-CMK, MANF, is unusual, since it is either retained or secreted, depending on the stress. Overexpression of MANF, or addition of recombinant MANF (rMANF) to culture medium, protected cardiomyocytes, while MANF knock-down increased simulated I & I/R-mediated cell death. Hypothesis: The specific hypothesis addressed in this proposal is that ischemia, which is known to activate ER stress, induces the ERS-CMK, MANF, in an ATF6-dependent manner, and acts intra- and extracellularly to protect the heart from ischemic damage. The Specific Aims that address this hypothesis are to: 1. examine expression of MANF in the ischemic mouse heart subjected to ATF6 gain- and loss-of-function, 2. determine the effects MANF gain- and loss-of-function in the ischemic heart, in vivo, and 3. dissect the functions of intra- and extracellular MANF, delineate the structural features of MANF required for its activities, and identify signaling mechanisms responsible for MANF function in cardiomyocytes.